The addRoutes method need to suport adding dynamic routes as children to an existing route

[hzsrc]

What problem does this feature solve?

The addRoutes method currently only suport add new routes to root routes. But if can not add routes as children to an existing route.

What does the proposed API look like?

router.addRoutes(routes: Array, parent ? : Location)

Like:
router.addRoutes([...childRoutes], '/main')

[posva]

Duplicate of #1156